Variable capacitor having a solid dielectric between its electrodes

ABSTRACT

A capacitor having a stationary electrode, a movable electrode, and a solid dielectric material between the electrodes. The solid dielectric occupies no more than a portion of the gap between the electrodes, and the remaining portion may be filled by a pressurized gas. Each electrode may be a series of spaced concentric tubular plates interleaved with the plates of the other electrode. The solid dielectric may be a layer covering some or all of the plate surface of one or both electrodes, and the dielectric layer may vary in thickness in the direction of relative movement of the electrodes.

United States Patent Goetzl 7 us]- 3,700,983 [451 Oct. 24, 1972 FOREIGNPATENTS ORAPPLICATIONS 1,130,842 211957 France ..317/253 In Manlio t n NPrimary Examiner-E. A. Goldberg 73 Assi nee: International Tele hone andTele- Attorney-C Coma! Remsen Paul 1 g graph commfimrNew York,:lemmlnger, Walter J. Baum and Thomas E. Kristoferson [22] Filed: June10, 1971 [21] Appl. No.: 151,658 v ABSTRACT A capacitor haying astationary electrode, a movable [52] US. Cl ..317/245, 317/251 e t nd aso d ect ic mate a between the [51] Int. Cl. ..l-l0lg 5/02 electrodes.The solid dielectric occupies no more than [58] Field of Search..3l7/245, 251, 253, 258 a portion of the gap between the electrodes,and the remaining portion may be filled by a pressurized gas.Rfiierences Cited Each electrode may be a series of spaced concentricUNITED STATES PATENTS tubular plates interleaved with the plates of theother electrode. The solid dielectric may be a layer covering 1,560,76111/1925 Cohen v ggme or all of the plate surface-of one or both e|ecFlSCh trodes v dielectric layer may vary in thickness 2,088,949 8/ 1937Fekete....'. .,....317/25 P in h direction of relative movement f theelec 2,137,135 11/1938 Fuwa ..317/253 X trodes" 3,217,217 11/1965Hoptroff ..317/253 3,366,852 1/1968 Goetzl ..317/245 13 Claims, 4Drawing Figures 1 g Z/ I 4 p fl '22 /7 l2], Z3

l l /a 20 /Z Y PATENTEDncI 24 m2 9 5.5: 255 at: 2552 a i INVENTOR:

run 4 x0 aazrzz VARIABLE CAPACITOR HAVING A SOLID DIELECTRIC BETWEEN ITSELECTRODES This invention relates to variable electrical capacitorswherein a movable electrode may be adjustably positioned with respect toa stationary electrode to vary the capacitance value of the capacitor.

In these capacitors, each electrode usually comprises a series of spacedmetallic plates interleaved with the plates of the other electrode. Theinterleaved plates are separated by a dielectric, which may be a vacuum,or

may be air or'some other gas at atmosphericor higher.

pressure. In some cases the capacitor is immersed in a liquid, usuallyan insulating oil, in which case the dielectric between the plates isthe liquid itself.

The value of the capacitance for a given set of plates, and a givenenmeshed area of the plates, is determined by the dielectric constant ofthe'dielectric between the plates. For equal geometric configurations ofthe electrodes, the capacitance varies directly with the valueof thedielectric constant. When the dielectric is vacuum the dielectricconstant is theoretically unity. With gas of any types or pressuresused, the dielectric constant is also unity for any practical purpose.When the plates lower capacitance value.

sent or much thinner when the capacitor is tuned to a Another object ofthe invention is to provide a variable capacitor in which the gapbetween the electrode plates is partially filled with a soliddielectric, the

FIG. 1 is a fragmentary longitudinal cross-sectional view showing theinterior of a capacitor made according to this invention; a

FIG. 2 is an enlarged view of a portion of FIG. 1 showing the electrodesin a differentzrelative position;

FIG. 3 is a view similar to FIG. 2 showing a different solid dielectriccoating onthe plates; and

are immersed in a liquid, the dielectric constant is usually 2 or more.The dielectric constant of a typical solid dielectric, such as ceramic,may be as high as about 9. w i

It is obvious that the use of a liquidor solid dielectric in the spacebetween the interleaved plates has the advantage of attaining a highercapacitance for a given number and size of plates and given spacingbetween them. However, for higher radio frequencies and high powerapplications the use of a liquid or solid dielectric has the veryobjectionable drawback of a large power loss in the dielectric. The heatdeveloped in capacitors employing liquid or solid dielectrics is highand they just cannot be used for voltages and powers above a certainlevel. On the contrary, the dissipation factor of capacitors with air,gas or vacuum between the plates is exceedingly low and the capacitorsremain cool up to much higher power levels, have a high quality factor(Q), and their tuning characteristics are much sharper.

It is an object of the invention to provide a variable capacitor havingincreased capacitance as compared to comparable conventional capacitors,but in which the dielectric losses are maintained within acceptablelimits.

This objective is achieved by providing a solid dielectric between theelectrode plates of the capacitor, the solid dielectric occupying only aportion of the gap between any two opposed plate surfaces. The soliddielectric should have a relatively low dissipation factor and a highdielectric constant.

In a number of applications, both the applied voltag and the frequencydecrease as the variable capacitor is tuned to higher capacitance valuesby increasing the enmeshed area of the interleaved plates. At lowerfrequencies and voltages the dielectric losses are much lower, sincethese losses decrease in direct proportion to the frequency and with thesquare of the voltage.

It is therefore another object of the invention to provide a variablecapacitor wherein the thickness of the solid dielectric between theelectrodes is a maximum when the capacitor is tuned to its maximumcapacitance value, and the solid dielectric is either abno. 4 is a viewsimilar to FIG. 2 showing Still another form of solid dielectriccoating. 7

The capacitor chosen to illustrate the present invention may be similarvto the capacitor shown and described in US. Pat. No. 3,366,852. Such acapacitor includes a housing composed of conductive metallic end caps 10and 11 (only a portion of end capll being shown in-FlG. l) and anintermediate cylindrical section 12 of insulating material such asrelatively thick ceramic. v

, The end cap 10 has aclosed end or neck 13 of 35 reduced diameter,interiorly threaded to engage with and rigidly support a metallic disc14 on which a series of stationary electrode tubes 15 are mounted. Thesetubes are concentric with one another and with the axis of the housing.Three tubes have been shown, but they may be more or less in number,depending upon requirements. They may have a thickness of the order of.04 incheach.

- Interleaved with the tubular electrode plates 15 is a similar series15 mounted on a disc-like carrier 17. This series of plates is preciselyfitted into the spaces between the plates 15 (the clearance being, e.g.,of the order of .03 inch) and this spatial relation must be maintainedduring reciprocatory movements of the carrier 17 and tubes 16in an axialdirection. To serve as a reliable and accurate guidance for thismovement, the carrier17 is provided with a cylindrical sleeve 18 coax-.ial with the housing axis and slidably engaging a fixed tubular guide20. The latter is secured to an insulating element 21 (composed forexample of ceramic) by means of a flange 22 on the guide 20 and afastener 23 screwed into the inner end of the element 21. At its outerend the element is similarly secured, as by a fastener 24, to the plate14.

As a result of this arrangement, the two sets of interleaved electrodetubes are both supported by the same end cap 10, one being rigidly andimmovably held, the other being supported for precise axial movement.

The movable electrode structure 16, 17 is reciprocated by an actuatorwhich may be an externally threaded rod (not shown) engaging theinternal screw threads of a cylindrical element 31 mounted for axialmovement only and operatively connected to the movable electrode of thecapacitor. A yoke 33 carried by the closed end 32 of cylinder 31,engaging a pin 34 carried by sleeve 18 serve as a flexible transmissionbetween the actuator and the movable electrode. Thus, rotation of theactuator produces axial movement of the movable electrode. Carrier 17which supports the plates 16 of the movable electrode is provided with asliding contact 35 adapted to press yieldably and resiliently againstthe inner surface of end cap 11.

The electrode plates and 16 have surfaces 39 and 40 (see FIG. 2),respectively, facing each other. Each pair of opposed surfaces areseparated by a gap 41 According to the present invention, each gap 41 ispartially occupied by a solid dielectric material. The most convenientway to introduce the solid dielectric into the gap 41 is by applying itas a coating to one or both opposed surfaces 39 and 40 of the electrodeplates. The coating may be applied in any suitable manner. Thus, forexample, a ceramic dielectric coating may be appliedby spraying theliquid ceramic on to the electrode I plates or dipping the plates into aliquid ceramic. In

either case, the ceramic hardens into a solid dielectric layer. If theplates are aluminum, a solid dielectric coating of alumina oxide may beapplied by anodizing the plates.

In the embodiment of the invention shown in FIGS. 1 and 2, a soliddielectric layer 42 covers a little more than one-half the area of eachplate surface 39 and 40, the remaining area being uncoated. The coveredarea is that portion of each surface closer to the plate carrier orsupport 14 or 17. When the electrodes are fully enmeshed, as shown inFIG. I, i.e., the capacitor is adjusted at its maximum capacitancevalue, a substantial portion, e.g., 80 90 percent, of each gap 41 isfilled with the solid dielectric 42. As the electrodes are moved awayfrom each other to reduce the capacitance value of the capacitor, itwill be apparent that theportion of gap 41 occupied by solid dielectric42 decreases. When the relative position of the electrodes shown in FIG.2 is reached, no solid dielectric whatsoever is present in gap 41. Thus,in this embodiment, the portion of gap 4l'occupied bysolid dielectric 42varies, at different relative positions of' the electrodes, between zeroand 90 percent or even more.

Since, as mentioned above, in some applications the voltage andfrequency involved are lower when the capacitance setting is high, thedielectric losses in the coating 42 are at acceptable levels when thecapacitor is in the position shown in FIG. 1. However, when highervoltage and frequency is involved, the capacitance setting will be lower(e.g., as shown in FIG. 2) and hence less or no solid dielectric ispresent in the gap 41, thereby minimizing or eliminating the lossesproduced by the solid dielectric.

It has been found desirable to fill the portion of gap 41 not occupiedby a solid dielectric with a gas under pressure, since the presence ofsuch a gas increases the breakdown voltage of the capacitor. The fillingof the housing with the desired gas under pressure may take placethrough the temporary nipple 51 formed on the end cap 10 (FIG. 11).After filling, the nipple is sealed off in any appropriate manner. Tofacilitate the entry of the gas into the interior regions, theattachment element I4 is preferably provided with at least one passage52.

In the alternative embodiment of the invention shown in FIG. 3, theparts are identical to those of FIGS. 1 and 2 except for the form of thecoating. Thus, stationary support 14' holds plates 15', and movablecarrier 17' holds plates l6. However, whereas coating 42 of FIGS. 1 and2 covers only a portion of each surface 39 and 40 and has a uniformthickness, each surface of plates 15' and 16 facing a surface of anotherplate is substantially entirely covered with a solid dielectric layerhaving a thickness which varies in the direction of relative movement ofthe electrodes. Thus, the coating portion 45 closer to the support orcarrier is thicker than the coating portion 46. Although in theconfiguration shown each dielectric layer has a single step 47, moresteps may be employed to vary the thickness in several stages, or thevariation may be continuous. It will be appreciated that with thisembodiment the portion of solid dielectric in each gap 41 decreases asthe capacitance value of the capacitor is decreased, as is true with theembodiment of FIGS. 1 and 2. However, in this case, a portion of the gap41 is occupied by solid dielectric at every setting of the capacitor.

In the embodiment of FIG. 4, the parts are identical to those of FIGS. 1and 2, except for the form of the coating. Plates 15" are mounted onstationary support l4' and plates 16" are mounted on movable carrier17".. Plates l5" carry no solid dielectric coating whatsoever. However,each surface of plates 16" facing a surface of plates 15" carries a thinsolid dielectric coating 49. This coating may have a uniformthicknessand can occupy about 10 20 percent of the gap 41". If a properdielectric material, such as alumina oxide, is used having a relativelylow dissipation factor and a high dielectric constant, its presenceincreases the capacitance of the capacitor without causing excessivedielectric losses.

The invention has been shown and described in preferred forms only andby way of example, but many variations may be made in it which willstill be comprised within its scope. For example, in any of theembodiments described above, the solid dielectric coatings illustratedmay be carried by the plates of one or both of the electrodes. Also, thethickness of the solid dielectric coating will be selected to occupyfrom 10 to percent of the gap between plates depending upon factors suchas the material of the coating and the application which the capacitoris to serve. It is understood, therefore, that the invention is notlimited to any specific embodiment, except as defined in the appendedclaims.

What is claimed is:

l. A variable capacitor comprising a gas-tight cylindrical housinghaving a stationary electrode, a movable electrode spaced from saidstationary electrode to provide a gap between said electrodes, means forvarying the position of said movable electrode with respect to saidstationary electrodes without varying the spacing between saidelectrodes so as to adjust the capacitance value of the capacitor, and asolid dielectric between said electrodes, said solid dielectricoccupying no more than a portion of the gap between said electrodes inall relative positions of said electrodes and a gas under pressurehigher than ambient pressure occupying the portion of said gap notoccupied by said solid dielectric, said solid dielectric having adielectric constant higher than that of said gas.

2. A variable'capacitor as defined in claim 1 wherein the portion ofsaid gap occupied by said soliddielectric is different atdifferentrelative positions of said electrodes.

3. A variable capacitor as defined in claim 2 wherein the portion ofsaid gap occupied by said solid dielectric is greater when saidelectrodes are positioned to yield a higher value of capacitance thanwhen they are positioned to yield a lower value of capacitance.

4. A variable capacitor as defined in claim 1 wherein I 5. A variablecapacitor as defined in claim 4 wherein the plates of said electrodeshave surfaces facing each other, and said solid dielectric layer coversonly a portion of at least one of said surfaces, said covered portionbeing such that in certain relative positions of said electrodes nosolid dielectric is present between said plates.

6. A variable capacitor as defined in claim 1 wherein each of saidelectrodes comprises at least one metallic plate, said plates havingsurfaces facing each other, and said solid dielectric comprises a layerof material bonded to only a portion of each of said surfaces, theportion of each surface carrying said solid dielectric facing theportion of the other surface which is not covered by said soliddielectric material.

7. A variable capacitor as defined in claim 1 wherein said soliddielectric occupies between 10 and 90 percent of the gap between saidelectrodes.

8. A variable capacitor as defined in claim 1 wherein said soliddielectric is alumina oxide (AL O 9. A variable capacitor as defined inclaim 1 wherein said electrodes are concentrically-arranged nestedcylindrical tubes.

10. A variable capacitor as defined in claim 1 wherein said stationaryelectrode is a series of spaced cylindrical tubes concentric with oneanother, and said movable electrode is a similar series of spacedconcentric tubes interleaved with the tubes of said stationaryelectrode.

11. A variable capacitor comprising a stationary electrode, a movableelectrode spaced from said stationary electrode to provide a gap betweensaid electrodes, means for varying the position of said movableelectrode with respect to said stationary electrode without varying thespacing between said electrodes so as to adjust the capacitance value ofthe capacitor, and a solid dielectric between said electrodes, saidsolid dielectric occupying no more than a portion of the gap betweensaid electrodes in all relative positions of said electrodes, each ofsaid electrodes comprising at least one metallic plate, and said soliddielectric comprises a layer of material bonded to theplate comprisingat least one of said electrodes, said solid dielectric varying inthickness so that the portion of said gap occupied by said soliddielectric is different at different relative positions of saidelectrodes.

12. A variable capacitor as defined in claim 11 wherein the thicknessvariation of said solid dielectric is such that the portion of said gapoccupied by said solid dielectric 18 greater when said electrodes arepositioned to yield a higher value of capacitance than when they arepositioned to yield a lower value of capacitance.

13. A variable capacitor comprising a stationary electrode, a movableelectrode spaced from said stationary electrode to provide a gap betweensaid electrodes, means for varying the position of said movableelectrode with respect 'to said stationary electrode without varying thespacing between said electrodes so as to adjust the capacitance value ofthe capacitor, and a solid dielectric between said electrodes, saidsolid dielectric occupying no more than a portion of the .gap betweensaid electrodes in all relative positions of said electrodes, each ofsaid electrodes comprising at least one metallic plate, said plateshaving surfaces facing each other, and said solid dielectric comprises alayer of material bonded to each of said surfaces, the thickness of eachsolid dielectric layer varying in the direction of relative movementbetween said electhickest portion of the other layer.

1. A variable capacitor comprising a gas-tight cylindrical housinghaving a stationary electrode, a movable electrode spaced from saidstationary electrode to provide a gap between said electrodes, means forvarying the position of said movable electrode with respect to saidstationary electrodes without varying the spacing between saidelectrodes so as to adjust the capacitance value of the capacitor, and asolid dielectric between said electrodes, said solid dielectricoccupying no more than a portion of the gap between said electrodes inall relative positions of said electrodes and a gas under pressurehigher than ambient pressure occupying the portion of said gap notoccupied by said solid dielectric, said solid dielectric having adielectric constant higher than that of said gas.
 2. A variablecapacitor as defined in claim 1 wherein the portion of said gap occupiedby said solid dielectric is different at different relative positions ofsaid electrodes.
 3. A variable capacitor as defined in claim 2 whereinthe portion of said gap occupied by said solid dielectric is greaterwhen said electrodes are positioned to yield a higher value ofcapacitance than when they are positioned to yield a lower value ofcapacitance.
 4. A variable capacitor as defined in claim 1 wherein eachof said electrodes comprises at least one metallic plate, and said soliddielectric comprises a layer of material bonded to the plate comprisingat least one of said electrodes.
 5. A variable capacitor as defined inclaim 4 wherein the plates of said electrodes have surfaces facing eachother, and said solid dielectric layer covers only a portion of at leastone of said surfaces, said covered portion being such that in certainrelative positions of said electrodes no solid dielectric is presentbetween said plates.
 6. A variable capacitoR as defined in claim 1wherein each of said electrodes comprises at least one metallic plate,said plates having surfaces facing each other, and said solid dielectriccomprises a layer of material bonded to only a portion of each of saidsurfaces, the portion of each surface carrying said solid dielectricfacing the portion of the other surface which is not covered by saidsolid dielectric material.
 7. A variable capacitor as defined in claim 1wherein said solid dielectric occupies between 10 and 90 percent of thegap between said electrodes.
 8. A variable capacitor as defined in claim1 wherein said solid dielectric is alumina oxide (AL2O3).
 9. A variablecapacitor as defined in claim 1 wherein said electrodes areconcentrically-arranged nested cylindrical tubes.
 10. A variablecapacitor as defined in claim 1 wherein said stationary electrode is aseries of spaced cylindrical tubes concentric with one another, and saidmovable electrode is a similar series of spaced concentric tubesinterleaved with the tubes of said stationary electrode.
 11. A variablecapacitor comprising a stationary electrode, a movable electrode spacedfrom said stationary electrode to provide a gap between said electrodes,means for varying the position of said movable electrode with respect tosaid stationary electrode without varying the spacing between saidelectrodes so as to adjust the capacitance value of the capacitor, and asolid dielectric between said electrodes, said solid dielectricoccupying no more than a portion of the gap between said electrodes inall relative positions of said electrodes, each of said electrodescomprising at least one metallic plate, and said solid dielectriccomprises a layer of material bonded to the plate comprising at leastone of said electrodes, said solid dielectric varying in thickness sothat the portion of said gap occupied by said solid dielectric isdifferent at different relative positions of said electrodes.
 12. Avariable capacitor as defined in claim 11 wherein the thicknessvariation of said solid dielectric is such that the portion of said gapoccupied by said solid dielectric is greater when said electrodes arepositioned to yield a higher value of capacitance than when they arepositioned to yield a lower value of capacitance.
 13. A variablecapacitor comprising a stationary electrode, a movable electrode spacedfrom said stationary electrode to provide a gap between said electrodes,means for varying the position of said movable electrode with respect tosaid stationary electrode without varying the spacing between saidelectrodes so as to adjust the capacitance value of the capacitor, and asolid dielectric between said electrodes, said solid dielectricoccupying no more than a portion of the gap between said electrodes inall relative positions of said electrodes, each of said electrodescomprising at least one metallic plate, said plates having surfacesfacing each other, and said solid dielectric comprises a layer ofmaterial bonded to each of said surfaces, the thickness of each soliddielectric layer varying in the direction of relative movement betweensaid electrodes, the thinnest portion of each layer facing the thickestportion of the other layer.